Procathepsin D is over-secreted by some human cancer cells. This enzymatically inactive precursor has been established as playing an important role in the development of several types of cancer. In the present investigation, we used both the isolated human procathepsin D and a synthetic 44 amino acid peptide corresponding to the activation peptide of procathepsin D to test their effects on the proliferation of lung cancer cells. We showed that both the procathepsin D and the activation peptide act as growth factors. In parallel, we also measured the secretion of procathepsin D by lung cancer cells and compared the secretion with invasiveness through Matrigel membrane. Our findings represent the first experimental data showing the direct effects of procathepsin D and its activation peptide on growth and invasiveness of lung cancer cells.